This invention relates to an improved dry spinning process for the preparation of flame-resistant acrylonitrile polymer fibers containing a chlorinated acyclic hydrocarbon and optionally antimony trioxide.
Many fabrics, particularly those used in children's sleepwear, are required by law to be flame resistant. Modacrylic fibers made from acrylonitrile polymers containing 35 or 40% by weight of one or more halogen-containing monomers have seen commercially sold as providing the required flame resistance. However, such fibers are relatively expensive to manufacture. Further, modacrylic fibers are not equivalent in all their properties to acrylic fibers.
Addition of chlorinated hydrocarbons to acrylonitrile polymers for the purpose of improving the flame resistance of acrylonitrile polymer fibers is known as is joint use of these chlorinated hydrocarbons and antimony trioxide.
It has been found that chlorinated hydrocarbons and antimony trioxide (and alternatively other additives such as tin, zinc or other antimony compounds, and other reagents known in the art to be synergistic with chlorine compounds in providing flame resistance) can be incorporated into spinning solutions of acrylic or modacrylic polymers. However, if conventional acrylonitrile polymers having an intrinsic viscosity of about 1.4 are used, only about 32% by weight chlorinated hydrocarbons based on fiber weight can be spun with any reasonable continuity of spinning. This amount of chlorinated hydrocarbon, even when supplemented with antimony trioxide, does not provide a high degree of flame resistance in that the fabrics containing such fibers do not pass the vertical flame test (for example DOC FF 3-71--Children's Sleepwear Test). In order to pass the vertical flame test, acrylonitrile polymer fibers should contain 33-43% by weight well dispersed chlorinated hydrocarbon, depending on the amount of chorine in the chlorinated hydrocarbon and the amount of antimony trioxide present.
This invention provides an improved process for spinning flame-resistant acrylonitrile polymer fibers wherein 33-43% by weight of a chlorinated acyclic hydrocarbon containing 60-73% by weight chlorine and 57-67% by weight of an acrylonitrile polymer having an intrinsic viscosity of 1.6-3 is dissolved in a suitable solvent to form a solution containing 25-31% by weight solids which solution is extruded under conventional dry spinning conditions to give fibers which are extracted, drawn and dried. Preferably, the spinning solution also contains antimony trioxide in an amount of 0.5-6% by weight of the total solids. It is preferred that the acrylonitrile polymer is an acrylic polymer having an intrinsic viscosity of 1.8-2.2. Most preferably, 35-40% by weight of a chlorinated acyclic hydrocarbon containing 66-70% by weight chlorine is used. Preferably, 60-65% by weight of the total solids in the solution spun is acrylonitrile polymer.
The chlorinated hydrocarbons employed in the present invention are low melting solids which are available commercially. They are made by chlorinating acyclic hydrocarbons to form a product which has an average of about 16-30, preferably 23-27, carbon atoms per molecule, as determined in the product as chlorinated. Since it is desired to introduce a substantial amount of organo-substituted chlorine in the mixture by the use of this additive, the minimum chlorine content of the chlorinated hydrocarbon is about 60% by weight; and the maximum useful chlorine content for dry spinning is about 73% by weight. The preferred starting materials are substantially straight-chain hydrocarbons and the preferred chlorine content is in the range of about 66-70% by weight. A highly suitable material is a chlorinated paraffin having a melting point of about 115.degree. C., containing approximately 25 carbon atoms, and having a chlorine content of about 67% by weight.
Any suitable inert organic solvent for acrylonitrile polymer may be employed. Preferred solvents are dimethylformamide and dimethylacetamide. Other examples include dimethylsulfoxide and butyrolactone. Normally the chlorinated hydrocarbon is mixed with the solvent first and brought into solution by heating and stirring. The appropriate amount of the acrylonitrile polymer, as further defined below, is then added and heating and stirring of the mixture is continued until a good solution of the components is formed.
The ratio of the amount of chlorinated hydrocarbon used to the amount of acrylonitrile polymer will depend upon the concentration of the chlorinated hydrocarbon desired in the fiber product. The higher the amount of chlorinated hydrocarbon in the fiber, the more flame resistant the fiber is. However, there is a limit to the total solids (polymer, chlorinated hydrocarbon and any synergistic additive) level at which a single phase can be maintained in the solution of the chlorinated hydrocarbon, acrylonitrile polymer, and inert organic solvent. Moreover, as the level of chlorinated hydrocarbon increases, the total solids level at which a single phase can be maintained decreases.
For instance, with acrylonitrile polymers having an intrinsic viscosity of 1.6 to 3 and dimethylformamide as solvent, when the chlorinated hydrocarbon amounts to 33% by weight of the fiber product, the total solids level in the solution which can be maintained as a single phase is only about 31% by weight. When the total solids level is 32% by weight and the chlorinated hydrocarbon comprises 33% or more by weight of the total solids, a second phase forms and the two-phase solution is not spinnable. When the chlorinated hydrocarbon is increased to 43% by weight of the fiber, the total solids level maintainable as a single phase is only about 25% by weight.
The acrylonitrile polymers used in the process of this invention are defined as long chain synthetic polymers composed of acrylonitrile units of the formula ##STR1## in the polymer chain, the intrinsic viscosity of the polymer being in the range 1.6-3 (preferably 1.8-2.2). Herein, the term "acrylonitrile polymer" includes the homopolymer of acrylonitrile (i.e., polyacrylonitrile) and copolymers of at least 60% by weight of acrylonitrile and up to 40% by weight of one or more suitable monoethylenically unsaturated monomers copolymerizable with acrylonitrile. The term "acrylic polymer" includes the homopolymer of acrylonitrile and copolymers of at least 85% by weight of acrylonitrile and up to 15% by weight of one or more suitable monoethylenically unsaturated monomers copolymerizable with acrylonitrile. Typical monomers which are copolymerizable with acrylonitrile are methyl acrylate, vinyl chloride, styrene, methylvinylpyridine, sodium styrenesulfonate, sodium 2-acrylamido-2-methylpropanesulfonate, and sodium methallysulfonate. Although the process of the invention is applicable to the modacrylic polymers containing 60-85% by weight acrylonitrile units, the preferred starting materials are the acrylic polymers.
In accordance with the present invention, it has been found that spinnable solutions of acrylonitrile polymer containing from 33 to 43% chlorinated hydrocarbon (based on total solids) can be prepared by using an acrylonitrile polymer having an intrinsic viscosity in the range of about 1.6 to 3. Best results are obtained by using an acrylonitrile polymer having an intrinsic viscosity of at least about 1.8, and polymers having an intrinsic viscosity above about 2.2 are generally more expensive to make in commercial practice; hence, the preferred range of intrinsic viscosity is from 1.8 to 2.2.
The polymer employed for dry spinning of conventional acrylic fibers in current commercial practice, which has an intrinsic viscosity of about 1.4, is not suitable for use as a starting material in the present invention. When acrylonitrile polymer having an intrinsic viscosity of only 1.4 is used, the maximum amount of chlorinated hydrocarbon which can be incorporated is only about 32% by weight (based on fiber weight). When more than about 32% by weight of the chlorinated hydrocarbon is added to the spinning solution, the total solids content of the spinning solution either becomes so high that the solution separates into two phases, or the concentration of acrylonitrile polymer in the solution must be reduced by adding more solvent. In either case, the solution obtained is unsuitable for good continuity of spinning.
Antimony oxide in the amount of 0.5-6%, based on the weight of the fiber product, is preferably added to the spinning solution to enhance the flame resistance of the fiber in cooperation with the chlorinated acyclic hydrocarbon. The optimum amount of antimony oxide is about 3% by weight, since higher amounts tend to deluster the fiber excessively while lower amounts do not confer as much flame resistance.